This invention relates generally to communication networks, and more particularly, to local-area networks having multiple nodes or stations that may transmit data to each other over the network. A number of schemes have been proposed for connecting the multiple stations together for this purpose. There are three basic configuration types that are commonly used. One type includes one or more common communication links, or buses to which each of the stations is connected. A message from a station is broadcast to all other stations, but only the intended destination station will pick up the message. Another type is a star or hub network in which each station can transmit its message to a central control unit, which then relays the message to the stations over the corresponding communication channels. The third type is a ring network, in which a message is relayed by each station to the next one in the ring, and the destination of the source station takes the message out of the ring.
Use of any such network has to be governed by a set of protocols or rules. The network protocols fall generally into two categories: contention schemes and non-contention schemes. In a contention scheme, stations may begin transmitting whenever they have a message to send, but may continue transmitting only if the network is not already in use. If such a "collision" is detected, all the stations involved in the collision stop transmission and later attempts must be made for access to the network. In a non-contention scheme, some form of time-division multiplexing is usually involved. For example, each station can be allocated its own time slot on a cyclic basis. Alternatively, a token is passed around to coordinate the message transmissions. In more complex non-contention schemes, a central controller may handle requests for network access by the respective stations, and access may be granted on some type of priority basis.
One problem with contention schemes in general is that there is the possibility of an unbounded transmission delay. If a station attempts to transmit but has to abort the transmission because other stations are also transmitting, there is always the possibility that the same thing will happen on the next attempt. In heavy traffic conditions, the transmission delay is completely without limit, and this is an unacceptable condition for most practical communications systems. In general, an ideal local area network should render high performance over a wide range of transmission data rates and traffic conditions.
Since many communications systems are now being planned or implemented in optical fiber form, another important requirement of an ideal local area network is that it be compatible with optical fiber technology. The ideal network should also be capable of handling continuous traffic from a station when required. In some applications, a station may have to transmit a continous sequence of data packets for some periods of time, to provide real-time control information, for example. An ideal network should be capable of handling this type of traffic, as well as ordinary traffic that is transmitted in accordance with some predefined set of priorities.
The ideal network should be designed in accordance with what is known as the over-commitment principle, that is, it should have a capacity only slightly greater than the average traffic load expected from all stations, and should provide some technique for smoothing out the occasional peaks or above-average traffic conditions.
Prior to this invention, no optical fiber local area network has provided all of these ideal features. A significant development in the field of communication networks was a system known as Ethernet, first described by R. M. Metcalfe et al. in a paper entitled "Ethernet: Distributed Packet Switching for Local Switching Networks," Comm. of the ACM, July 1976, pp. 395-404. Its fiber optics variation, Fibernet, is described by E. G. Rawson and R. M. Metcalfe in "Fibernet: Multimode Optical Fibers for Local Computer Networks," IEEE Trans. on Communications, July 1978, pp 983-90. Both Ethernet and Fibernet have the disadvantage that a station may have to wait for an indefinite time to transmit a packet of data, because of possible repeated collisions with other packets accessing the network simultaneously. This possibility of an unbounded transmission delay renders both Ethernet and Fibernet unsuitable for the transmission of real-time or continuous traffic. Moreover, as the data rate of the transmission medium increases, the efficiency for maximum throughput of these systems decreases.
Further developments, using a unidirectional bus, are known as Express-net and C-net. Express-net is described in a paper presented by L. Fratta et al., entitled "The Express-Net: A Local Area Communications Network Integrating Voice and Data," at the International Conference on Data Communication Systems: Performance and Applications, Paris, September, 1981. Express-net has a relatively high efficiency and a relatively low bounded network delay, making it suitable for continuous or real-time traffic. C-net is another network with a unidirectional data bus. It avoids some of the difficulties of Express-net, but has a maximum network delay almost double that of Express-net.
Fasnet is yet another network configuration, described in a paper by J. O. Limb et al. entitled "Description of Fasnet--A Unidirectional Local-Area network," The Bell System Technical Journal, Vol. 61, No. 7, September, 1982. All of these systems, including Express-net, C-net, and Fasnet, have a network delay in the order of NT.sub.p, where N is the number of stations and T.sub.p is the packet transmission time, i.e., for the one message packet to pass a fixed point in the network. This is also true of a system referred to by the name D-net, described in a co-pending patent application by the present inventor, Ser. No. 449,083, filed on Dec. 13, 1982, and entitled "Communication Network and Method For It's Use". If a continuous-traffic station has a need to transmit a packet of data every t.sub.g seconds, then the number of stations that any of these networks can serve is limited to: EQU N*=t.sub.g /T.sub.p.
Otherwise, the quality of service to the continuous-traffic station cannot be guaranteed. To assure service to the continuous-traffic station, the channel capacity of the network has to be larger than the possible maximum or peak load of the network, and this is clearly wasteful since the average traffic load from all stations can be relatively low.
In addition, the architectures of the systems mentioned above are not ideally compatible with optical fiber technology. The systems each require several optical taps and couplings to be made for each station. This severely limits the number of stations that can be attached to the network, and adds difficulty to the design of receivers at the stations, since the receivers have to have a large dynamic range. Another disadvantage is that a single malfunctioning station can, by continued transmission, prevent or degrade the performance of the network.
Some of the disadvantages described are avoided by a network that employs star couplers instead of input and output buses as the basic configuration. The general principle of this type of system is that each of the stations is connected to the coupler by its own two transmission lines, one for sending and the other for receiving. A message from a particular station is broadcast to all stations by the coupler, but as in any network, there must be an effective technique to resolve conflicts between simultaneous requests for transmission.
One network of this type is disclosed in U.S. Pat. No. 4,428,046, issued in the names of Chari et al. In the Chari et al. network, a number of stations, referred to as "subsystems," are connected to a star coupler by sending and receiving transmission lines, and the star coupler includes contention circuitry for ensuring that no more than one selected subsystem can pass a message through the star coupler at any particular time. The contention circuitry operates in accordance with a set of predefined and fixed station priorities. If two or more messages from different subsystems are received simultaneously, the one from the subsystem with the highest priority is the one that is processed first. As in some of the other network configurations discussed, the subsystems or stations whose messages were not accepted for transmission must retransmit at some later time. The possible network delay is, therefore, unbounded.
Another drawback to the approach of the Chari et al. network is that, although one station may monopolize the network and exclude other stations, it is not possible to provide a predetermined continuous transmission capability, for stations that have a need to transmit real-time data in a continuous fashion. Yet another drawback is that the efficiency of the network decreases as the round-trip transmission time increases. Finally, as the number of stations is increased, the star coupler circuitry becomes increasingly complex, and less suitable for mass production.
It will be appreciated from the foregoing that there is still a need for improvement in the field of local area network systems, particularly of the type suited to optical fiber technology. In particular, what is needed is a fiber-optic communications network that has high performance characteristics, specifically a low bounded delay and a high efficiency, over a wide range of data transmission rates. Also, the system should be able to accommodate continuous-traffic stations as well as stations having normal sporadic traffic demands, and should have a capacity of only slightly greater than is necessary to accommodate average traffic load conditions. Ideally, the system should also be unaffected by the malfunction of any particular station. The present invention provides a novel solution to these requirements.